You presume MTBE was necessary in the first place.
When O2 sensors appeared in automobiles in 1980 the EPA got a wild-eyed idea that they could make existing engines run cleaner than when qualified for emission testing by oxygenating the fuel to make it run richer. The O2 sensor only detects the stoichiometric optimum mixture and can't be made to peak for any other point. They ran field tests in the early 1980's with first generation O2 sensor engines and "proved" their concept. And then "reformulated gasoline" became law. Initially only in high pollution regions, of course, then the world!
Shortly after it was well known how reformulated gasoline from some regions of the country got horrible MPG in vehicles which did much better elsewhere. Before ethanol was allowed without prominent notice on the gas pump. Back when it was called Gasohol.
The problem was by 1990 combustion engineers did what government bureaucrats thought impossible: they figured out how to get around the O2 sensor limitations. Digital computer control was relatively cheap and easy to create complex maps and recalculate on the fly. Previously the carburetor and ECU were slaves to what the O2 sensor said. But then they learned they could just use the O2 sensor now and then for a calibration point, and run whatever they wanted at other times. Suddenly V6's and V8's started getting 30 MPG on the highway cycle.
EPA was none too happy, this was an early scenario of what later happened with VW's diesel emissions cheating. Automakers were deliberately running leaner, and dirtier, under steady state conditions such as found on open highways. They made no apology. Unlike VW, emissions were under the regulations. Its just that the EPA couldn't tune the emissions by revising the fuel's oxygen content.
To this day government being government will not admit reformulated gasoline no longer works.